As radio frequency signal detection techniques for detection by a detector diode, thesubsequente well known techniques as disclosed in Japanese Patent 2561023, Japanese Patent 2605827, Japanese Unexamined Patent Application Publication No. 4-291167, Japanese Patent 3202624, and Japanese Unexamined Patent Application Publication No. 10-56333, for example. The above conventional techniques include many devices of the type that, for example, uses multiple stages of amplifier circuits to detect a detection signal and to perform level determination. In activation signal output circuits of such a type, since signals are DC, the respective stages are configured in a direct connection pattern.
However, the circuit configuration of such a direct connection pattern of the stages causes problems when configuring, for example, a circuit that enables stable detection of a radio frequency signal of a very low level of, for example, −60 dBm. More specifically, noise generated in amplifiers in the respective stages indicates a large value in a near-DC zone, and also DC components of the noise are to the amplifiers in the subsequent stages. This becomes the cause of erroneous detection of the radio frequency signal and the cause of disabling sensitivity improvement. In addition, it is difficult to form, for example, all the transistors for constituting the amplifiers in the respective stages to have the same characteristics, such that offset in the DC potential can occur in the respective interstages. When the offset potential is transferred to the subsequent stage, also the signal levels of, for example, the detection signal and reference signal are offset, such that there can occur a case in which the transistors are saturated to thereby disable executing real detection signal amplification. Of course, such a phenomenon can be a cause of erroneous operation.
The present invention is made to solve the problems described above. Accordingly, an object of the invention is to realize an activation signal output circuit having very high sensitivity; and more particularly, a primary object of the invention is to enhance the S/N ratio and detection accuracy, for example.